The invention relates to a tool, especially a cutting insert for the machining of metallic workpieces, which is composed of a hard metal base body, a cermet base body, a ceramic base body or a steel base body, especially a high speed steel base body, and at least one layer deposited thereon of which the sole layer or, in the case of multiple layers, the outermost layer or a layer lying under the outermost layer contains molybdenum sulfide.
The invention relates further to a method of producing the molybdenum sulfide containing coating by means of CVD (chemical vapor deposition).
The coating of substrate bodies of hard metal, a cermet, a ceramic or steel with hard materials or hard material mixtures or ceramic materials is known in the art. Especially used are carbides, nitrides, carbonitrides, oxycarbonitrides, oxides and/or borides of at least one of the elements of Groups IVa through VIa of the periodic system as, for example, TiC, TiN, Ti(C,N) or also ceramic materials like Al2O3 or ZrO2. For large cutting cross sections and/or large chip cross sections high temperatures develop at the cutting edge of the machining tool especially with workpieces which are difficult to machine, thereby further increasing the wear and contributing in part to edge breakage.
To minimize the temperatures which arise in machining at the tool edge, so-called cooling lubricants are customarily used which, however, contain environmentally contaminating and toxic substances and which must be disposed of at high cost.
A further possibility, which can drastically reduce the use of coolant lubricant substances is the principle of so-called minimum quantity lubrication whereby very reduced amounts of coolant lubricant substances are supplied with point precision by fine passages to the contact locations between the tool edge and the workpiece. Apart from a high process technology and apparatus cost, these approaches do not eliminate the disposal problem but only minimize it.
On the other hand, so-called solid dry lubricant films are known in the state of the art, for example, molybdenum is disulfide which is known from DE-A-24 15 255 in which it is proposed to increase the adhesion to support surfaces by the so-called sputtering of an MoS2 or WS2 based layer with a thickness of 1-2 xcexcm and then to apply a dry lubricant layer preferably of MoS2 or WS2 also on the base layer in a known manner by a polishing application.
In the East German Patent DD 202898 it has been proposed to coat cutting tools, stamping tools and machining tools with layers or the like of molybdenum disulfide by sputtering whereby the layer thickness is in the nm range. The hard material layer applied to a substrate body by sputtering should have a hexagonal lattice structure.
EP 0 534 905 A2 also proposes the PVD (plasma vapor deposition) coating of machining tools inter alia with molybdenum disulfide.
It has, however, been found that MoS2 layers produced by cathode atomization (sputtering) adhere poorly to the substrate body or on the outermost hard material layer applied to the substrate body or have frequently an unsatisfactory orientation of the MoS2 crystallites. To obtain reduced friction coefficients, the hexagonal components of the MoS2 layer should be so oriented that the hexagonal base planes lie parallel to the surface. In this manner, the oxidation resistance of the layer is also improved.
In the DE 35 16 933 A1 it has been proposed to set the ratio of the water vapor partial pressure to the coating rate at 10xe2x88x927 mbar/1.6 mgmxe2x88x922 s31 1, which in practice has been found to be obtainable only with high technical cost. Apart from the requirement customary for the PVD process for pure gas atmospheres, i.e. gas atmospheres which do not contain impurities, there is a further drawback of the PVD process in that a targeted particle stream is produced from the target source to the substrate, whereby the substrate must be rotated about three axes to produce a uniform coating.
Furthermore, WO 97/04142 describes a method for the uniform coating of MoS2 and a metal or a metal compound like titanium or titanium nitride by use of two targets by means of a sputtering process.
While the MoS2 is relatively soft and not especially wear resistant, MoS2 coatings have a significant effect since in the use of such a single or outermost MoS2 layer, while rapidly rubbed off at the contact location of the tool with the workpiece still noticeably reduces the friction force for a certain period because of a penetration of MoS2 particles from the edge inward (entrainment).
In DE 44 14 051 C1, a low-friction layer composite for components of metal materials, to which class tools can also belong, is proposed in which a high frequency magnetron atomization results in a nanodispersion forming an intermediate layer of Cr3Si or of chromium to which an MoSx, layer is applied by means of high frequency magnetron atomization.
U.S. Pat. No. 5,709,936 describes a coated substrate body which is comprised of a layer of a hard material phase and a solid lubricant phase. The proportion of the hard material phase should amount to 50 to 90 volume %. The solid lubricant phase should preferably be composed of molybdenum sulfide. The coating is applied by means of CVD, whereby the substrate body is heated to a temperature between 350xc2x0 C. and 850xc2x0 C. Then a gas mixture of Ti((CH3)2N)4, NH3, MoF6 and H2S is admitted, whereby the two phase layer TiN-MoS2 is formed.
In DE 38 34 356 A1, the deposition of thin molybdenum sulfide layers from a gas phase is described, the gas phase containing molybdenum hexacarbonyl, hydrogen sulfide and argon as carrier gas.
In the previously known CVD depositions, only relatively thin molybdenum sulfide layers can be produced in which the hexagonal crystallite planes lie parallel to the substrate surface and at the same time are dense. With longer layer growth periods, morphologies develop in which the hexagonal molybdenum crystallites are oriented perpendicularly to the substrate surface. With molybdenum sulfide layers of greater thickness, there is ready ablation of the layers which because of the poor space filling also are associated with a limited wear resistance.